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...when I did that and then inserted, my whole line moved when actually I just want to add more nodes and not move anything....

Then pick your new points on the polyline. Use the NEA object snap.

If you are trying to densify one or more polylines, then you will need to purchase or write your own routine. By densify, I mean adding many vertices based on some criteria, like one vertex every "X" units, or "X" number of vertices total in a given polyline...

"If you are trying to densify one or more polylines, then you will need to purchase or write your own routine."

that's exactly what I'm trying to do.

By purchase a routine , do you mean purchase a program that layers the nodes close together? And by write my own routine , you mean that I would hand click each node so that they are layered so close together?

what I'm looking to do is have as many nodes as I can, layered as closly together as I can.
Our Cnc machine is half way broken and doesn't understand curves right now. The only way the machine can draw a curve is by reading where the nodes are placed . If there is a gap in the nodes that peice of the wood doesn't get cut.
Right now we are drawing in autocad and then having to transfer the image to another program that places the nodes as close as it can together.

I'm trying to cut out that last step. Each of these suggestions would work in there own way but it's still more work than transfering to the other program.... I'm hoping to be able to find something in autocad that allows me to automaticly have those nodes placed there ; laying there as neighbors as close as can be for a clean cut...

what I'm looking to do is have as many nodes as I can, layered as closly together as I can.

Then you are looking for a "densify polyline" routine. You should have stated this up front instead of saying "How do I insert a new grip or node in to an existing polyline?" - which made it look like you only wanted to insert a single node on a single polyline....
This functionality s not included with vanilla AutoCAD.
You need a 3rd party routine.
You can write something yourself if you are so inclined, or purchase a supported program, or look for an unsupported freebie.